http://cdn.arstechnica.net/wp-content/uploads/2012/07/A222-223-640x541.jpg Map of galaxy clusters Abell 222 and 223, with a filiment of dark matter connecting them. The contours and the blue color represent the distribution of mass, as determined by weak lensing.
J√∂rg Dietrich, University of Michigan/University Observatory Munich

Simulations of the Universe on the largest scales show an unexpected resemblance to nerve cells in the human brain, with galaxy clusters playing the role of the cell body and thinner filaments of matter linking them like axons. Galaxy surveys (such as the Sloan Digital Sky Survey, or SDSS (http://www.sdss.org/)) show that galaxies do cluster like our simulations predict. But the filaments that should connect them have been harder to find. Most of the mass in the Universe is dark matter‚??material that neither emits nor absorbs light‚??and filaments are predicted to be mostly dark matter: no galaxies, little hot gas.

Einstein's general theory of relativity, however, tells us mass affects the path of light, and a group of astronomers have identified a dark matter filament by measuring this effect. J√∂rg P. Dietrich et al. measured the slight distortion and magnification of background galaxies by the mass in a filament between two galaxy clusters, Abell 222 and 223. By comparing the distortions with X-ray measurements, the researchers determined that the filament contains very little hot gas, lacks galaxies entirely, and is invisible in optical wavelengths.

These observations lend strong support to the theory that the Universe is built on a web of dark matter that has drawn in visible structures like galaxies and clusters.